Method and apparatus for the excitation of a plasma

ABSTRACT

A method for the excitation of a plasma in which a gas is subjected to an electric field by a plurality of electrode systems. According to the invention at least two separate electrode systems are used which are power supplied from separate generators with the same frequency, the voltage variations of the generators being shifted in phase relative to each other such that a voltage zero never occurs at the same time in two of the electrode systems, a kind of rest period also occurring, in which there is no significant potential difference between the phases. As a result a pulsating plasma is obtained, as the plasma is generated by the potential difference between the phases. When there is no significant potential difference between the phases, an added substance may interact with its own functionality.

This application claims the benefit of International Application NumberPCT/DK00/00018, which was published in English on Jul. 27, 2000.

TECHNICAL FIELD

The invention relates to a method for the excitation of a ply in which agas is subjected to an electric field by means of at least two separateelectrode systems, which are power supplied from separate generators ofthe same frequency, the voltage variations of the generators beingshifted in phase relative to each other.

BACKGROUND ART

It is often advantageous to use a low-frequency alternating voltageinstead of DC voltage for generating a plasma in order to prevent theformation of constant sparks between the electrodes. In this connectionlow-frequency signifies frequencies above 2 Hz, but below 10 Hz. One ofthe advantages of using low-frequency alternating voltage is that animpedance matching between the generator and the plasma is not required.Another advantage of using low-frequency alternating voltage is thatreactive losses in power supply lines, feed-throughs and the like can beignored, which simplifies the equipment design significantly.

The known systems use an electrode or an electrode system supplied byone alternating voltage. However this configuration only render fewpossibilities for adjusting the intensity and homogeneity of the plasmato the particular need

Furthermore it is known from EP 0831679 to discharge the voltages forgenerating the plasma from one phase by means of a power portion, theindividual portions of the power being substantially equally shifted inphase relative to each other.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is to show how to provide AC electrodesystems which are able to meet several different requirements ofintensity and homogeneity in the generated plasma.

A method of the above type is according to the invention characterisedby choosing the phases and/or the amplitudes in an asymmetrical way soas to obtain a kind of a periodical rest period in which there is nosignificant potential difference between the phases, voltage zero neveroccurring simultaneously in two of the electrode systems.

As a result a pulsating plasma is obtained, as the plasma is powered bythe potency difference between the phases. In the part of the period inwhich no particular potential difference exist between the phases, anadded substance may interact with its own functionality.

In this context generator signifies a voltage/power source, in which thephase of the alternating voltage on the output is substantiallyinsensitive to the load caused by the plasma. A generator may be atransformer connected directly to one of the three phases of the mainsand zero or between two of the three phases of the mains. A generatormay also be formed of another voltage source rendering an oscillatingsignal as an output signal, the frequency of which being determined by alocal control circuit.

By using two electrode systems a plasma with relatively low intensity isobtained in relation to a plasma with many electrode systems that areall supplied with the same voltage.

Moreover according to the invention the electrodes may be placed along acircle, the plasma being generated in the center of the circle. As aresult the plasma is generated in an inhomogeneous zone adjacent theelectrodes, while the central part of the chamber is filled withhomogenous “diffusion” plasma. The conditions in the plasma in thehomogenous central part are such that reactions, which normally wouldnot be produced in conventional plasma equipment, can be obtained, asthe molecules are only broken into smaller fragments in this area. Theplasma is thus “gentle” towards an added substance, such as an addedmonomer.

In another configuration the electrodes are placed along a cylindricalbody which in turn is encased in a tube, the plasma being generatedbetween the electrodes and the interior of the tube. During thegeneration of the plasma the tube is thus able to rotate slowly.

In a particularly advantageous embodiment the phase shift between twophase may be Φ, where

0.5<Φ+1·π<2.5

where 1 is a positive integer. By selecting an asymmetrical phase shift,the intensity of the plasma may be varied during the oscillation period,eg by displacing the two phases by 30° such that a pulsating plasma isgenerated, the plasma being generated by means of the potentialdifference between the phases. As a result an added monomer is able tointeract with its own functionality, ie to obtain a form of equilibriumin the part of the period when there is no significant potentialdifference between the phases.

At higher voltage amplitudes of one of the phases it is possible toperform tasks, in which for instance a special geometry of the blankimplies special requirements. As an example it is often necessary in aninternal plasma processing to place an electrode inside the tube or hoseand to impress a higher voltage on this electrode than on the electrodesgenerating the rest of the plasma.

Optionally according to the invention three or more discrete electrodesystems are used, which are power supplied from separate generators, atleast two of the said generators being of the same frequency and used togenerate an AC plasma, the voltage variations of the generators beingshifted in phase relative to each other, and at least one of theelectrode systems comprising at least two electrodes. The third phasemay thus for instance be used for depositing a metal coating by acathode sputtering process. Optionally this phase can also be forcleaning the surface of a blank.

The invention further relates to a use of the method according to theinvention for excitating a fluorescent material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below with reference to theaccompanying drawings, in which

FIG. 1 shows a cylindrical reactor for the excitation of a plasma bymeans of a plurality of electrode systems, each electrode systemcomprising a number of electrodes placed along the inner surface of thecylindrical reactor,

FIG. 2 shows a reactor with an electrode configuration wherein theelectrodes are arranged in the centre of the cylindrical reactor, andthe plasma is generated between the electrodes and the inner surface ofthe reactor.

FIGS. 3 and 4 show a part of a system for generating a two-step process,in which an AC plasma is generated during a first process by means oftwo or more phases and the surface of a blank is cleaned during a secondprocess, and

FIG. 5 shows a system for continuous coating of silver by cathodesputtering.

BEST MODE FOR CARRYING OUT THE INVENTION

The cylindrical reactor shown in FIG. 1 for the excitation of a plasma,wherein a gas, e.g., argon, helium, or nitrogen, is subjected to anelectric field generated by a plurality of electrode systems, saidreactor comprising a plurality of electrodes arranged along the innersurface of the reactor. Having many electrodes of the same phase 1 and2, respectively, the plasma is generated in an inhomogeneous zone 3adjacent the electrodes, while the central part of the reactor chamberis filled with a homogenous “diffusion” plasma. The conditions in theplasma in the homogenous central part 4 are such that reactions, whichnormally would not be produced in conventional plasma equipment, areobtained, as the molecules are only broken into smaller fragments inthis area. The plasma is thus “gentle” towards an added substance, suchas an added monomer.

In an optional embodiment shown in FIG. 2, the electrodes 1′ and 2′,respectively, are arranged in the centre of the cylindrical reactor andthe plasma is generated substantially between the electrodes 1′, 2′ andthe inner surface of the reactor. The shown electrode configurations areonly some of the many possible symmetrical and asymmetricalapplications.

The generators operating the reactors may be provided in different ways.In this context generator signifies a voltage source, in which the phaseof the alternating voltage on the output is substantially insensitive tothe load caused by the plasma. A generator may be a transformerconnected directly to one of the three phases of the mains and zero orbetween two of the three phases of the mains. A generator may also beformed of another voltage source rendering an oscillating signal as anoutput signal, the frequency of which being determined by a localcontrol circuit.

According to the invention two, three or more electrode systems may beused which are supplied with power from separate generators having thesame frequency, but shifted in phase relative to each other. The phaseshift between two phases is Φ, where for instance 0.5<Φ+1·π<2.5 and 1 isa positive integer. Other ratios are, however, also possible. The phaseshift between the individual phases may vary. However, by choosing anasymmetrical phase shift, the intensity of the plasma may be variedduring the oscillation period, eg by arranging three (out of three)phases with a phase shift of 30°, a pulsating plasma is obtained, theplasma being generated by the potential difference between the phases. Amonomer (for instance acryl or vinyl added to the reactor) will theninteract with its own functionality in the portion of the period inwhich there is no significant potential difference between the phases.This reactor is activated by the radicals formed during the plasmaperiod in the surface of the monomer. An example thereof is plasmapolymerization of vinylpyrolidol, where a constant plasma tends todestroy the ring structure in the monomer.

A higher voltage (amplitude) of one of the phases furthermore makes itpossible to solve tasks in connection with blanks of a special geometry.During a plasma processing of the interior of thin flexible tubings itis often necessary to place an electrode inside the tubing and impress ahigher voltage amplitude on this electrode than on the electrodesgenerating the other plasma.

It it often desired to combine plasma modification or plasmapolymerization of a surface with other processings either before orafter the plasma processing, eg cleaning (before) or metallisation(after).

In a particularly advantageous embodiment shown in FIGS. 3 and 4 it isshown how some of these processes may be combined in one and the samevacuum chamber.

This combination of processes may be obtained by using three or moreseparate electrode systems power supplied from separate generators. Atleast two of these generators operate at the same frequency and are usedto generate an AC plasma.

The voltage variations of the plasma generators are, however, shifted inphase, preferably such that a voltage zero never occurs at the same timein these two electrode systems. Each electrode system comprises n, melectrodes, where n, m is greater than or equal to 1; at least one ofthe electrode system must, however, comprise at least two electrodes.

By selecting few (for instance two) electrode systems a plasma withrelatively low intensity is obtained in relation to a plasma with manyelectrode systems, when the same voltage is impressed on the electrodesystems.

The combined process steps are powered by a generator which may have thesame or another frequency than the plasma generators or by a DC voltage.This process step is placed in or adjacent the AC plasma such that thisprocess step may exchange free electrons and/or ions with the AC plasma.When intensifying the combined process steps (with magnets or highvoltage) a very high electron or power density may be obtained which canbe used for instance for cleaning undesirable material, such as greaseor oil from the surface of a blank or to power a cathode sputteringprocess, in which a metal or another material is to be applied to theblank from a cathode sputtering electrode. If the blank is oblong, eg inform of a cable in unrolled state, both the unwinding and the windingtake place in the vacuum chamber per se. The cylindrical vacuum chamber,of which only a disc-shaped portion is shown in FIGS. 3 and 4, hastypically a diameter of 55 mm and a length of 130 cm. The pressure inthe chamber is preferably between 0.01 and 10⁴ Pa.

As mentioned above, the term generator signifies a voltage source, inwhich the phase of the alternating voltage on the output issubstantially insensitive to the load caused the plasma. In this contexta generator may be a transformer connected directly to one of the threephases of the mains and zero or between two of the phases. A generatormay also be formed of another voltage source rendering an oscillatingsignal as an output signal, the frequency of which being determined by alocal control circuit.

The plasma electrode systems used are supplied with power from separategenerators, the output voltages thereof operating at the same frequency,but being shifted in phase relative to each other. The phase shiftbetween two phases may be Φ, where for instance 0.5<Φ+1·π<2.5 and 1 is apositive integer. Other ratios are, however, also possible. In systemscomprising more than two phases, the phase shift between the separatephases may differ.

As mentioned above, by choosing an asymmetrical phase shift, theintensity of the plasma may be varied during the oscillation period, egby phase shifting three (out of three) phases with 30° a pulsatingplasma is obtained, the plasma being generated by the potentialdifference between the phases. As a result a monomer, which has beenadded to the reactor chamber, is able to interact with its ownfunctionality in the part of the period in which there is no significantpotential difference between the phases.

EXAMPLE 1

By combining an area containing a “soft” plasma (many electrodes perphase) with for instance two phases including one electrode having athird phase, optionally amplified by means of magnetic fields, in anadjacent area, a situation arises, in which a polymer, such as PTFE, inthe “soft” area may be modified in the surface by means of an addedmonomer, whereafter the polymer is brought adjacent the third phase, inwhich a coating of metal—confer FIG. 3—such as platinum or silver isapplied by means of a cathode sputtering process (at 6), where theelectrode material is the metal to be sputtered on the polymer. In thisconfiguration it may be an advantage that the impressed voltage at thethird phase (at 6) is not the same as at the other phases. It is alsonecessary that the electrodes are not made of the same metal, but ofmetals with different sputtering rates.

EXAMPLE 2

A cleaning (at 7) of for instance a continuous carbon fibres for processfacilities (eg lubricants) may be combined with plasma polymerization ofa coating (eg fluorine polymer for PTFE or PVDF), confer FIG. 4.

EXAMPLE 3 Polymerization of Vinylpyrolidon

Plasma polymerization of vinylpyrolidon is normally not possible withoutcausing a ring opening of the five-member ring at the nitrogen atom,whereby the plasma-polymerised layer is provided with other propertiesthan polyvinyl pyrolidon prepared at conventional radicalpolymerization. The reason why is that other (several) processes takeplace in the plasma than the processes necessary for a radicalpolymerization, and which unfortunately destroy the monomer in anundesirable manner. Polyvinyl pyrolidon is inter alia used for makingsurfaces more bio-compatible.

By allowing periods of time, in which a potential difference existsbetween the electrodes and whereby a plasma is generated—alternatingwith periods without potential difference, the monomer is (nearly only)able to perform a radical polymerization in the voltage-free periodsbased on the radicals generated in the periods with plasma.

The differences between the various plasma polymerisations appear fromthe FTIR spectra of NaC1 crystals which are plasma-polymerised withvinyl pyrolidon on the surface.

If ring opening takes place, the carbonyl peak changes from 1706 cm⁻¹ to1692 cm⁻¹ or lower at the same time as an amine peak occurs at about3300 cm⁻¹.

The best results are obtained at a pressure of 0.15 mbar, a voltageamplitude of 160V, a phase shift of the three phases of 30°, 30° and300° and with argon as plasma gas (in addition to vinyl pyrolidon).Optionally the best results are obtained at two phases with a shift of120°/240°, a voltage amplitude of 240 V and/or the same conditions aswith three phases. In both cases a processing time of 60 seconds rendersa complete covering of the surfaces.

EXAMPLE 4 Continuous Silver Plating of Polyethylene

A (profiled) band (cable) of polyethylene (or another polymer such asPP, EVA, PVDF, PTFE, FEP . . . ) is wound through a two-step processing,in which a “binder” is initially plasma-polymerised on the polymersurface, the silver being immediately subsequent thereto cathodesputtered on said binder.

The plasma for the plasma polymerization is driven by a “two-phased”plasma. Argon and ethylcyanoacrylate are used as plasma gasses in thepolymerization.

A third phase is connected to a tubular cathode sputter 14, throughwhich the polymer band is inserted—confer FIG. 5. The cathode sputter 14is formed of 30 mm long silver tubing 9 enclosed by four ring magnets 10separated by PFTE discs 11. The cathode sputter is furthermore encasedby an insulating tube and a water cooler for conducting the producedheat away from the outer surface thereof.

The phase shift between the three phases is 120°.

At a pressure of 0.2 mbar and a voltage amplitude of 400 V between thetwo plasma phases and 280 V on the sputtering phase, a covering silverlayer was obtained having a good adherence at a band velocity of 3m/min. It was subsequently possible to deposit copper on this silverlayer by means of an electroplating process.

EXAMPLE 5 Plasma Coating of Pigments with Polyacrylic Acid

In order to adapt pigments (eg carbon black) to aqueous media such aspaint and ink (eg for InkJet), it is desirable that the pigment surfaceis highly hydrophilic in order to obtain a good dispersion of thepigment in the aqueous medium.

This object is accomplished by placing 100 g of pigment in a rotatingcylindrical drum, in which a set of electrodes are arranged about acylindrical axis. The pigment is arranged in the space between therotating drum and the electrodes. The diameter of the drum is 25 cm, andthe distance between the drum and the electrodes is 5 cm, the length ofthe drum being 30 cm. The electrodes are connected to two power supplieshaving a phase shift of 120°. At a voltage amplitude of 320-450 V acrossthe electrodes and a pressure of 0.2 mbar, a plasma of a suitableintensity is generated between the electrodes and the drum. Argon andacrylic vapour is fed to the drum in the ration 3:1, the drum rotatingwith five rpm for exposing the pigment to the plasma. A processing timeof five minutes is required in order to coat the entire pigment surfacewith polymerised acrylic acid.

EXAMPLE 6 Polyphase AC Plasma as (Flexible) Light Source

A set of electrodes is arranged on the inner surface of a flexible,transparent tube. This can for instance be obtained by pultruding thinconductors (eg 12 aluminium conductors with a diameter of 0.5 mm)lengthwise of the inner surface of a silicone or a PU tube (with aninside bore of 30 mm) such that an angle of 140° of the stripped metalof the conductors abuts the interior of the tube. The electrodes areconnected to two or three phases AC having a phase shift of 120°.

The inner surface of the tube is coated with one layer of fluorescentmaterial (like a fluorescent tube) or the fluorescent material (powder)is mixed into the polymer prior to pultrusion.

The pressure in the tube is reduced to 1 mbar. The gas used may forinstance be argon or atmospheric air.

At a voltage amplitude of from 120 to 280 V a plasma is generated in thetube. The plasma excites the fluorescent material such that light isemitted from the tube. The fluorescent material is chosen so as toprovide the light from the tube with the desired colour.

What is claimed is:
 1. A method of producing a pulsating plasma of lowintensity, the method comprising subjecting a gas to an electric field,the, electric field being generated by means of at least two separateelectrode systems, at least one of the two separate electrode systemscomprising at least two electrodes, wherein said at least two electrodesystems are being power supplied from separate generators of the samefrequency and of voltage variations shifted in phase relative to eachother so that a rest period of no significant potential differenceexists between said shifted phases.
 2. A method according to claim 1,wherein at least one of the separate generators is in the form of atransformer connected directly to either one of the three phases of themains and zero, or between two of the phases.
 3. A method according toclaim 1, wherein the electrical field is generated by means of at leasttwo separate electrode systems, the voltage variation of the generatorsbeing shifted in phase by about 180° relative to each other.
 4. A methodaccording to claim 1 or 2, wherein the phase shift between two phases isΦ wherein 0.5<Φ+I·π<2.5 and I is a positive integer.
 5. A methodaccording to claim 1, wherein the electrodes are placed along acylindrical body, which in turn is encased in a tube, the plasma beinggenerated between the electrodes and the interior of the tube.
 6. Amethod according to claim 5, wherein the electrical field is generatedby means of at least three separate electrode systems.
 7. A methodaccording to claim 1, wherein the voltage variation of the generatorsare shifted asymmetrically relative to each other.
 8. A method accordingto claim 1, wherein the electrical field are generated by means of twoseparate electrode systems, each of the systems comprises at least twoelectrodes.
 9. A method according to claim 8, wherein said electrodes ofthe two separate electrode systems are being connected alternatingly toshifted phases.
 10. A method according to claim 8 or 9, wherein saidelectrodes of the electrode systems are placed alternately to surround aspace wherein the plasma is generated.
 11. A method according to claim1, wherein the voltage amplitude used at one of the phases is higherthan those voltage amplitudes used at other phases.
 12. A methodaccording to claim 1, further comprising generation of an additionalprocess selected from a group consisting of a cleaning process and asputtering process, wherein said additional process is conducted in theplasma, and wherein a further cleaning/sputtering electrode systemoperated at an electron or power density used for cleaning undesirablematerial or for sputtering a metal to a blank is placed in the plasma.13. A method of plasma polymerization of a monomer onto a surface, themethod comprising providing a pulsating plasma of low density as definedin claim 1, adding the monomer to said pulsating plasma, and exposingthe surface to said monomer containing pulsating plasma.
 14. A methodaccording to claim 13, wherein the monomer is acrylic acid vinylpyrrolidon, or ethylcyanoacrylate.
 15. An apparatus for producing apulsating plasma of low intensity, the apparatus comprising: (a) areaction chamber for containing a plasma; (b) a gas supply for supplyinga gas to said reaction chamber; (c) at least two separate electrodesystems for generating an electrical field in said vacuum chamber, saidat least two separate electrode systems having at least one electrodesystem comprising at least two electrodes; and (d) separate power supplygenerators for providing power supplies of same frequency, said at leasttwo separate electrode systems being power supplied from said separatepower supply generators of the same frequency and of voltage variationsshifted in phase relative to each other so that a rest period of nosignificant potential difference exists between said shifted phases. 16.An apparatus according to claim 15, wherein said reaction chamber is avacuum chamber, said vacuum chamber having a pressure in the rangebetween 0.01 and 10⁴ Pa.
 17. An apparatus according to claim 15 or 16wherein said vacuum chamber is a flexible, transparent tube.
 18. Anapparatus according to claim 15, further comprising a rotatingcylindrical drum for containing a sample.
 19. An apparatus according toclaim 15 wherein said supply of gas comprises an addition substance,preferably a monomer.
 20. An apparatus according to claim 19, whereinsaid supply of gas comprises a gas selected from a group consisting ofargon, helium, and nitrogen, said additional substance being acrylicacid, vinyl pyrrolidon, or ethylcyanoacrylate.
 21. An apparatusaccording to claim 15 wherein said at least two separate electrodesystems consist of three or more separate electrode systems powersupplied from separate generators, at least two of said generatorsoperating at the same frequency and used to generate an AC plasma. 22.An apparatus according to claim 21, further comprising a generatorhaving same or different frequency as said plasma generators, or agenerator generating a DC voltage.
 23. An apparatus according to claim22 further comprising density intensifiers comprising magnets or highvoltage generators.
 24. An apparatus according to claim 15, 21, or 22,further comprising a cleaning unit, a cathode sputtering electrode, or acombination thereof.
 25. An apparatus according to claim 24, whereinsaid cathode sputtering electrode is a silver tubing.